The present invention relates to manufacturing an optical fiber preform. More particularly, it relates to a method and to apparatus for manufacturing an optical fiber preform.
European patent application EP-A1-0 450 465 describes manufacturing a preform from a so-called xe2x80x9cprimaryxe2x80x9d preform which goes to make the light-guiding portion of the resulting optical fiber, by a technique of external deposition on the sides thereof, e.g. the plasma buildup technique. The final preform obtained in this way, more generally referred to simply as the xe2x80x9cpreformxe2x80x9d, is for use in manufacturing an optical fiber by fiber-drawing.
Patent application EP-A1-0 875 489 describes a method of manufacturing an optical fiber preform which comprises rotating a cylindrical primary preform about its axis, with the preform and a plasma torch being moved relative to each other in translation and with glass powder being introduced into the plasma flame, the glass powder being accelerated prior to penetrating into the plasma flame by an accelerator gas which is introduced into the flow of glass powder falling under gravity. The acceleration is adjusted by adjusting the flow rate of the accelerator gas. The accelerator gas can be subjected to pretreatment such as heating or having a doping material introduced therein. The axis of the plasma flame and the axis of the preform are offset by a distance which is generally a function of the acceleration of the powder.
With a method of the kind described in patent application EP-A1-0 875 489, it is difficult to optimize the efficiency of silica deposition because it is also necessary to manage removal of the heat generated by the plasma torch, and that can impede deposition of silica on the preform. In order to be able to remove heat efficiently, it is necessary to extract air and thus to disturb the flow of the accelerator gas. Consequently, the powder grains arrive in uncontrolled and disordered manner on the preform, and the advantage of using an accelerator gas is thus compromised or even lost.
The method of the invention seeks mainly to increase the deposition efficiency of the external deposition method. It also seeks to remove plasma torch heat under good conditions without disturbing the introduction of powder into the plasma flame.
To this end, the invention provides a method of manufacturing an optical fiber preform, the method comprising rotating a primary cylindrical preform about its own axis X, establishing relative displacement in translation between the preform and a plasma torch, and introducing a glass powder into the plasma flame, the glass powder being accelerated prior to penetrating into the plasma flame by means of an accelerator gas introduced into the flow of glass powder advancing under gravity, wherein the manufacturing method takes place in an enclosure and wherein glass powder introduction takes place in a gaseous atmosphere at a pressure lower than the pressure outside the enclosure, the pressure difference between said enclosure and the medium outside the enclosure being regulated.
The pressure outside the enclosure, or the pressure of the medium outside the enclosure, is generally the pressure at which the glass powder is stored outside the enclosure prior to being introduced into the plasma flame.
In general, the pressure difference is regulated to a value that usually lies in the range xe2x88x920.3xc3x97102 Pascals (Pa) to xe2x88x922.xc3x97102 Pa. For example it can be xe2x88x920.9xc3x97102 Pa.
Suction pressure regulation is generally performed on the basis of calibration on preforms used as test pieces. Such reference suction pressure is a function mainly of the various parameters of the system. For calibration purposes, it is therefore necessary with one or more test preforms to evaluate various suction pressure values relative to accelerator gas flow rate for a given external atmosphere (temperature and humidity), for a given type of preform, and for glass powder of given grain size.
The invention also provides apparatus for manufacturing an optical fiber preform, the apparatus comprising a plasma torch, means for causing a primary preform to rotate about its own axis, means for establishing relative displacement in translation between said preform and said torch, and feed means for feeding glass powder to the flame of said plasma torch, said feed means itself including means for injecting acceleration gas, said apparatus including an enclosure for preform manufacture, which enclosure is connected to a pressure regulator unit for regulating the pressure inside the enclosure.
Advantageously, said regulator unit enables the pressure inside the enclosure to be maintained at a pressure lower than the pressure outside the enclosure.
The pressure outside the enclosure is more generally the pressure at which the glass powder is stored outside the enclosure prior to being introduced into the plasma flame.
Generally, said pressure regulator unit maintains the pressure difference inside the enclosure (where said pressure difference is the pressure inside the enclosure minus the external pressure) at a value that usually lies in the range xe2x88x920.3xc3x97102 Pa to xe2x88x922.xc3x97102 Pa, e.g. xe2x88x920.9xc3x97102 Pa, relative to the pressure outside the enclosure.
The suction pressure is generally regulated on the basis of calibration on preforms that are used as test pieces. Such a reference suction pressure is a function mainly of various parameters of the system. For calibration purposes, it is therefore necessary to use one or more test preforms to evaluate various suction pressure values relative to accelerator gas flow rate, for a given outside atmosphere (temperature and humidity), for a given type of preform, and for glass powder of given grain size.
In an embodiment, said unit includes a device for measuring the pressure difference between the enclosure and the medium outside the enclosure.
In an embodiment, optionally different from the preceding embodiment, said unit further includes means for blowing air into the enclosure.
In an embodiment, optionally different from the preceding embodiment, said unit further includes means for extracting air from the enclosure to the medium outside the enclosure.